The present invention relates to TACAN antenna arrays. Specifically, a highly efficient feed network is described which reduces RF power losses to a TACAN antenna array.
For many years, TACAN navigation systems have been used on board aircraft for determining an aircraft position. On the ground, a transmitting antenna array is provided which generates a rotating azimuth pattern. The rotating pattern has a basic cardioid shape, and includes nine minor lobes, as well as the cardioid main lobe. The nine minor lobes constitute an envelope modulation of the azimuth pattern superimposed on the cardioid pattern. The cardioid pattern rotates at 900 RPM. Aircraft position is determined by noting the position of the rotating pattern with respect to the aircraft when the pattern is pointing to a true North reading. By noting the position of the cardioid and the nine lobes with respect to the aircraft, it is possible to determine the bearings of an aircraft.
The required antenna pattern having the nine minor lobes superimposed on the rotating cardioid is derived by a multi-element circular array. The array is driven with a feed network which excites each element of the array with energy having a distribution to produce the required cardioid and nine lobes.
A common feed network used to drive the multi-element circular antenna array is the Butler matrix network, which receives two pairs of continuously phase shifted RF signals, and a larger amplitude constant phase RF signal. The constant phase RF signal generates an omni-directional azimuth pattern. The first pair of the continuously phase shifted secondary RF signals modulates the omni-directional pattern to provide the cardioid pattern which rotates at the rate of 15 Hz. and the second pair are phase shifted at nine times the 15 Hz. rate to produce the rotating nine lobe envelope function.
The Butler matrix is advantageous in that the amplitude modulation envelope results from combining phase displaced energy radiated from each element of the array. However, to derive the typical 32 RF signals for driving 32 TACAN array elements, the matrix requires a considerable number of phase shifting, power splitting and power combining elements. The number of elements in the matrix increases the net insertion loss and then lower loss transmission medium must be used in order to meet gain specifications of the TACAN array. This imposes an increased cost of manufacture for the individual matrix elements. Additionally, if the large amplitude constant phase zero mode omni-directional signal must pass through all of these network elements, the cost of manufacture is increased because the elements must handle higher power signals.